Method and apparatus for growing crystals

ABSTRACT

A POWDER DISPENSING ASSEMBLY FOR DELIVERING POWDERED CRYSTAL GROWING CONSTITUENTS TO THE CRYSTAL FORMING ZONE OF A CONVENTINAL VERNEUIL CRYSTAL GROWING APPARATUS. THE ASSEMBLY COMPRISES A DUAL-CHAMBERED POWDER DISPENSING CONTAINER ROTATABLE ABOUT A HORIZONTAL AXIS AND ADAPTED TO BE POSITIONED VERTICALLY ABOVE A VERNEUIL BURNER FOR THE INTRODUCTION OF POWDERED FEED CONSTIUENTS TO THE CRYSTAL GROWING ZONE THEREOF.

1971' J. L. SAMPSON METHOD AND APPARATUS FOR GROWING CRYSTALS Filed Oct.18, 1968 4O A rvilllo United States Patent O 3,558,196 METHOD ANDAPPARATUS FOR GROWING CRYSTALS John L. Sampson, Lexington, Mass.,assignor t the United States of America as represented by the Secretaryof the Air Force Filed Oct. 18, 1968, Ser. No. 768,627

Int. Cl. B65g 53/40 US. Cl. 302-59 4 Claims ABSTRACT OF THE DISCLOSURE Apowder dispensing assembly for delivering powdered crystal growingconstituents to the crystal forming zone of a conventional Verneuilcrystal growing apparatus. The assembly comprises a dual-chamberedpowder dispensing container rotatable about a horizontal axis andadapted to be positioned vertically above a Verneuil burner for theintroduction of powdered feed constituents to the crystal growing zonethereof.

BACKGROUND OF THE INVENTION The present invention relates to an improvedapparatus for growing crystals. More particularly, this inventionconcerns itself with an improved apparatus for effecting crystal growthin accordance with the well-known Verneuil fusion process.

With the present interest in laser technology, the need for lasercrystalline materials possessing unique and powerful characteristics hasbecome a problem of paramount importance. Crystalline materials with aclearly defined interface or boundary, such as ruby against sapphire,possess the unique characteristics that make these materials especiallyadaptable for laser use. However, these crystals are especiallydifiicult to produce using known methods such as the Verneuil fusionprocess. In this process crystalline growth is effected by introducingpowdered crystalgrowing constituents into a heated crystal growing zone.The powder is dispensed from a container positioned vertically above thezone and becomes entrained within a stream of oxygen gas as it descendsinto the crystal growing zone. The powdered constituents are heated inthe growing zone to a molten condition by an oxy-hydrogen flame. Themolten material falls onto a crystal growing support means where itadheres and grows into a crystal. As the crystal forms, it is slowlydrawn away from the flame so that only the growing surface of thecrystal is maintained in the molten condition. Crystal growth continuesas additional powdered material falls onto the molten surface.

One of the primary disadvantages encountered in using this well-knownprocess, however, involves the step of replenishing the powderdispensing container with powdered material without interrupting the gasflow and temperature conditions utilized during crystalline growth.Also, it is extremely difiicult to produce crystalline materials whichpossess a boundary or interface because of the relative difficulty ofmaintaining the crystal growing surface in a molten state while changingthe chemical composition of the powdered feed constituents. In order tochange the type of powder being fed to the growing zone, it becomesnecessary to stop the flow of oxygen carrier gas through the powderdispensing assembly to the growing zone. This stops crystal growthbecause the molten state cannot be maintained at the growing surface ofthe crystal. The resulting interruption of the required growingconditions prevents extended growth on the crystal surface because theseconditions cannot be maintained during the step of replenishing theassembly with powdered constituents.

With the present invention, however, the problems prevalent with priorart powder dispensing assemblies has been overcome by providing a devicewhich comprises a multiple-chambered powder dispensing container whichis rotatable about a horizontal axis such that either chamber may beplaced in a vertical position for the delivery of powdered material tothe heated crystal growing zone of a conventional Verneuil burner. Theutilization of the powder dispensing assembly of this invention providesa simple, economical and rapid means for accomplishing crystallinegrowth without interference or interruption of the crystal growingconditions encountered in the crystal growing furnace. It also providesa simple and convenient means for replenishing the container withpowdered feed constituents without interrupting crystal growingconditions. This provides for the continuous growth of a single crystalor the growth of crystalline structures possessing well-definedboundaries.

SUMMARY OF THE INVENTION In accordance with the present invention, ithas been found that rapid and extended crystalline growth can beaccomplished in a Verneuil fusion burner by utilizing a powderdispensing assembly which comprises a doublechambered powder container.The chambers are positioned in a back-to-back relationship with respectto one another and are rotatable about a horizontal axis. The assemblyis positioned vertically with respect to the position of the heatedcrystal growing zone. The rotatability of the chambers permits a rapidinterchange of their positions and allows for the replenishment ofpowdered material without disturbing the growing conditions neededduring crystal growth. Changing the chemical composition of the powderedfeed constituents to produce crystal boundaries, such as ruby againstsapphire, or supplying large amounts of powder for extended crystalgrowth are also accomplished with great facility.

Accordingly, the primary object of this invention is to provide animproved apparatus for growing crystalline structures at hightemperatures using the Verneuil fusion process.

Another object of this invention is to provide an apparatus foreffecting the extended growth of crystalline structures from powderedcrystal growing materials without interrupting the growing conditionsencountered during crystal formation.

A further object of this invention is to provide an apparatus foralternately feeding different powdered materials to the crystal growingzone of a fusion-type crystal growing apparatus without interfering withthe growing conditions utilized during crystal growth.

The above and still further objects and advantages of this inventionwill become apparent upon consideration of the following detaileddescription thereof when taken in conjunction with the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

The figure represents a fragmentary cross-sectional view of the powderdispensing assembly of this invention.

Referring to the drawing, the figure shows details of a powderdispensing assembly in accord with the invention. The apparatuscomprises an inner assembly 10 and an outer hopper 12 which is adaptedto be positioned vertically and connected at 14 to the crystal growingzone of a conventional Verneuil fusion apparatus, not shown. The innerassembly comprises a double-chambered cylindrical container 16, pivotedat 18 to a yoke 20 and rotatable about a horizontal axis as shown byarrows 22. The yoke 20 is attached to an inner cover 24 by suitablemeans such as by welding. The inner cover 24 is attached to an outercover 26 by means of adjusting screws 28,

while the outer cover 26 is in turn attached to hopper 12 by bolts 30.The inner cover 24 is sealed against the outer cover 26 by means of anO-ring seal 32. The outer cover 26 is in turn sealed against the hopper12 by an O-ring seal 34. The O-rings 32 and 34 effectively seal theupper interior portion 36 of the hopper 12 from the atmosphere. Thelower wall of the interior portion 36 is conical in shape with its innersurface 70 tangential to the circle through which the container 16rotates as shown by arrows 22.

The container 16 comprises a first powder containing chamber 40 and asecond powder containing chamber 42 positioned vertically back-to-backand separated by a wall 44. At least one chamber is always in positionto dispense a powdered feed constituent through outlet means 46 to thecrystal growing zone of a Verneuil burner, not shown. A gas inlet 48 isprovided to permit the introduction of an oxygen carrier gas stream inwhich the powdered constituents are entrained while being introducedinto a growing zone. A porthole 50 is provided to permit entry into theinterior portion 36 of the hopper 12 in order to fill the chambers 40 or42, as the case may be, with powdered feed constituents. As shown in thedrawing, the container 16 is in a raised position free to rotate aboutits horizontal axis. This axis runs through wall 44. Two bar magnets 52and 54 are affixed to the outer wall 56 of container 16 in parallelconfiguration. These magnets are acted upon by a permanent magnet, notshown, which would be positioned exterior to the outer hopper 12.Rotation of the container 16 is effected by rotating the outer permanentmagnet which in turn acts upon the two bar magnets 52 and 54 to causethe desired rotation.

Each end of chambers 40 and 42 is enclosed respectively with fine meshscreens 58 and 60 which are held in place by spring clips 62 and 64. Thechambers can be filled with powdered material by removing the springclip and screen. The container 16 is also provided with two O-ring seals66 and 68 which provide for an effective seal between the container 16and the inner wall surface 70 of the outer container 12. The surface 70lies in a tangential relationship to the circle through which the innercontainer 16 rotates. Lowering the inner assembly by means of adjustingscrews 28 to a position indicated by dotted lines 72 and 74 places theO-ring seal 66 against the inner wall surface 70. This creates aneffective seal and isolates the lower portion 38 of hopper 12 from itsupper portion 36. Powder from the lower chamber 42 can then be shaken bythe impact of agitator 76 through the fine mesh screen 60. The powder isentrained by a carrier gas stream introduced at inlet 48 and carriedthrough outlet 46 to the crystal growing zone of a conventional Verneuilburner, not shown.

The entire powder dispensing assembly and the Verneuil burner form aclosed system. As a consequence, the required temperature conditions andgas flow rates can be maintained without interference during theformation of crystal structures.

When chamber 42 empties itself of powder, crystal growth can be extendedby merely raising the inner assembly 10, rotating the container 16 suchthat the chamber 40 is now in the lower position while empty chamber 42assumes the upright position. The assembly 10 is then lowered by screws28 against wall surface 70 so that O-ring seal 68 can provide a sealbetween the upper portion 36 and lower portion 38. The powder in chamber40 can then be shaken by agitator 76 and carried through outlet 46 to aburner while simultaneously introducing powder into chamber 42. Thereplenishment of chamber 42 is accomplished by opening porthole 50removing spring clip 64 and screen 60 and then filling the chamber withpowder. After filling the chamber 42, the screen 60, spring clip 64 andporthole 50 are replaced to once again create a seal against the 4atmosphere. Chamber 42 is then in position to be rotated about ahorizontal or central axis whenever desired.

From a consideration of the above, it can be seen that the presentinvention provides for extended crystalline growth by the powderedfusion method. Growth is accomplished without interfering With crystalgrowing conditions. Also, boundary layers in crystalline structures canbe easily formed by employing powdered materials of differing chemicalcompositions in the separate powder containing chambers of the assemblyof this invention.

For example, the operation of the powder dispensing assembly may becarried out by placing aluminum oxide powder with no trace impurities inchamber 42. The assembly is activated by shaking the chamber 42 by meansof the agitator 76 and oxygen gas is introduced through inlet 48 tocarry the undoped aluminum oxide to a burner of a conventional Verneuilapparatus. This provides for the growth of a clear, sapphire crystal.While the sapphire crystal is growing, chamber 40 is filled withaluminum oxide powder containing trace amounts of chromium oxideimpurities. After sufiicient growth has been accomplished, and withoutinterrupting growth conditions, chamber 40 is rotated about a centralaxis to a downward position. The chromium oxide doped aluminum oxidepowder is then carried to the burner to effect the growth of a rubycrystalline structure on the surface of the already formed sapphirestructure. This forms an ultimate crystalline structure having a welldefined boundary interface of ruby on sapphire. Growth temperatures ofabout 2030 C. with gas flow rates of about 4 liters per minute for theoxygen carrier gas, 15 liters per minute for the oxygen and 25 litersper minute for the hydrogen are generally untilized to form thecrystals. The particular growth parameters required, however, willdepend on the particular materials employed and the type of structure tobe formed. These parameters are well known and form no part of thisinvention.

The powder dispensing assembly of this invention can be adapted for usewith any of the presently known modifications of the well known Verneuilfusion apparatus and the stoichiometry of the crystalline compounds thusformed can be closely controlled by adjusting the composition of thepowders to be placed within the dual chambers. This control can beeffected without interference with the crystal growing conditions.

No description has been given relative to the crystal growing furnacewhich would be employed with the present invention since this element isquite conventional. However, it would be connected to the powderdispensing assembly described herein by conventional connect ing meanspositioned between the powder dispensing outlet and the crystal growingzone.

The invention has been described with particular reference to a specificembodiment thereof. It is to be understood, however, that thedescription of the invention is for the purpose of illustration only,and it is not intended to limit the invention in any way.

What is claimed is:

1. A powder dispensing assembly for use with a fusiontype crystalgrowing apparatus comprising:

an outer container sealed at one end and having outlet means located atthe bottom thereof,

an inner dispensing assembly positioned within said outer container andincluding multiple chambers for powder, each chamber having an outlet,

means for successively positioning the outlet end of each of saidchambers toward the outer container outlet,

sealing means adjacent to the outlet of each chamber,

means for raising and lowering said inner dispensing assembly forsealing each chamber outlet with said outer container thereby separatingsaid outer container into a lower and upper portion with said lowerportion located adjacent to said outer container outlet means,

and inlet means for introducing a carrier gas into said lower portion ofsaid outer container.

2. A powder dispensing assembly in accordance with claim 1 whichincludes means in said upper portion of said outer container for gainingaccess to said inner dispensing assembly.

3. A powder dispensing assembly in accordance with claim 1 wherein saidchambers are mutually opposed and are remotely caused to rotate about acentral axis.

4. A powder dispensing assembly in accordance with claim 1 includingmeans to vibrate said inner dispensing assembly.

References Cited UNITED STATES PATENTS ANDRES H. NIELSEN, PrimaryExaminer US. Cl. X.R.

